The invention relates to a system for encoding data presented at one end of a transmission channel with a first coding device and for decoding the data encoded by the first coding device at the other end of the transmission channel with a second coding device.
Such a system is disclosed by EP 0 232 043 A2. This known system comprises a scrambler (the first coding device) at one end of the transmission channel for encoding incoming data (the data presented) and a descrambler (the second coding device) at the other end of the transmission channel for decoding the data encoded by the scrambler and for generating outgoing data. Said incoming data comprises words synchronously multiplexed in the frames. To indicate the frame timing, some of these words are so-called synchronisation words which must not be encoded. For this purpose, the scrambler comprises a frame-finding circuit which detects the synchronisation words in the incoming data and which, if a synchronisation word is detected generates a signal in response to which the detected synchronisation word is not encoded. At the same time, the descrambler also comprises a frame-finding circuit which detects the synchronisation words in the outgoing data and which, if a synchronisation word is detected, generates a signal in response to which the detected synchronisation word is not decoded. The descrambler furthermore comprises shift registers which, if the descrambler is unable to synchronise with the frames as a consequence of a fault in the transmission channel, alters the phase of the encoded data until the descrambler is again in a state which is synchronised with the frames. The effect of faults in the transmission channel on the system is reduced by using said shift registers.
Such a system has the disadvantage that the incoming data has to contain, or has to obtain, synchronisation information (the synchronisation words) since the descrambler is unable to get back to the state which is synchronized with the frames after getting out of synchronisation with the frames, and faults in the transmission channel can have a long-lasting disadvantageous effect on the decoding.